Catalytic cracking is a widely used process for conversion of relatively heavy hydrocarbons to lighter components. This process is particularly useful for the conversion of gas oil, topped crude, and the like, into components boiling primarily in the gasoline boiling range. For the catalytic cracking process, fluidized catalysts, generally silica based catalysts, are used. These catalysts may contain some material of the zeolitic structure or other additives and components. After the cracking catalyst has been used in the reactor, the coke formed on the catalyst is burned off generally in a separate regenerator and in a fluidized bed operation. In this regeneration, the catalyst activity is restored.
In the catalyst regenerator, the so-called "afterburning" constitutes a common and well known problem. This afterburning is caused by the presence of enough oxygen in the dilute phase of the regenerator to combust carbon monoxide, a product of the incomplete combustion taking place in the dense phase of the regenerator located below the dilute phase. This afterburning in the dilute phase is very undesirable because (1) very high temperatures can be caused by the afterburning, which in turn can cause damage to the cyclones, the vessel itself and the related equipment, (2) elevated temperatures can damage the catalyst, (3) the excess heat of the afterburning is not likely to be recovered and (4) the afterburning constitutes a safety hazard.
For controlling the afterburning, it has been suggested in the prior art to adjust the regeneration air rate. This procedure is rather involved because the air for the regenerator is normally provided from a large air blower that is ideally operated at constant, full speed and cannot readily be controlled. Further, operation of the air blower at less than full capacity is a waste which could be utilized in obtaining greater feed throughput, higher conversion and/or greater selectivity by better catalyst regeneration or higher circulation rates. Other methods for controlling the afterburning include adjusting the catalyst circulation rate, the oil feed rate and the injection of an external gas into the regenerator. These methods are not entirely satisfactory because they are either slow in response, result in lower conversion or lower production rates or are expensive.